The invention relates generally to a process for producing brown covering layers on semifinished products made of copper, especially for rolled tapes and sheets used in roofing and facing.
Under normal atmospheric conditions, a very adhesive and resistant covering layer of copper oxide forms on the surface of bare metal copper. This oxide film, which at first is very thin, stabilizes the surface of the copper material with respect to the effects of the atmosphere. Ideally, the slow further development of the oxide layer, which results from the continued chemical reaction of the copper with moisture and atmospheric oxygen, gradually forms a uniform brown coloring (brown patina), so that the surface of the copper increasingly loses its metallic shine. With the passage of time, the brown covering layer becomes progressively darker and turns into an anthracite brown. This is the final state that usually sets in on perpendicular building surfaces, such as on an outerwall facing. In the case of sloped roof surfaces, the roof layer changes in color by reacting with the substances contained in the atmosphere such as sulphur dioxide, carbon dioxide and chlorides, to form alkaline copper compounds, until the patina-green that is typical of copper is attained.
Under certain atmospheric conditions, however, the formation of the brown covering layer can be considerably delayed and also accelerated in spots, resulting in a relatively long wait until the copper surface becomes uniformly discolored. Deviations from a uniform shade are especially noticeable in the initial weathering stage. First, irregular dark spots and/or stripes form over and over again on the copper surface. As the weathering action continues, however, these color variations noticeably decline due to atmospheric influences.
This invention is directed to the development of a process by which a uniform and very adherent brown covering layer (brown patina) can be produced, if desired on a large scale, on the surface of a semifinished product made of copper. This covering layer will not become damaged or peel off when it is further processed as may be required in building construction.